Chapter 9Chapter 9Pathways that Harvest Chemical Pathways that Harvest Chemical

EnergyEnergy

Main Fuel Molecule: GlucoseMain Fuel Molecule: Glucose

Fuels: Molecules whose stored energy Fuels: Molecules whose stored energy can be released for use.can be released for use.

The most common fuel in organisms is The most common fuel in organisms is glucose. Other molecules are first glucose. Other molecules are first converted into glucose or other converted into glucose or other intermediate compounds.intermediate compounds.

Burning or metabolism of glucose:Burning or metabolism of glucose:

Glucose metabolism pathway traps the Glucose metabolism pathway traps the free energy in ATP:free energy in ATP:

How is Glucose Used to Make EnergyHow is Glucose Used to Make Energy

energyfreeOHCOOOHC 2226126 666

ATPenergyfreePADP i

How is Glucose Used to Make EnergyHow is Glucose Used to Make Energy

ΔΔGG is the change in free energy is the change in free energy ΔΔGG from from complete combustion complete combustion of of

glucose glucose = –686 kcal/mol= –686 kcal/molHighly exergonic; drives endergonic Highly exergonic; drives endergonic

formation of many ATP molecules.formation of many ATP molecules.

Three metabolic pathways are involved in Three metabolic pathways are involved in harvesting the energy of glucose:harvesting the energy of glucose:

GlycolysisGlycolysis: : glucose is converted to glucose is converted to pyruvatepyruvate

Cellular respirationCellular respiration: aerobic and : aerobic and converts pyruvate into converts pyruvate into HH22O, COO, CO22, and ATP, and ATP

FermentationFermentation: anaerobic and converts : anaerobic and converts pyruvate into lactic acid or pyruvate into lactic acid or ethanol, COethanol, CO22,, and ATPand ATP

The Big PictureThe Big PictureIf OIf O22 is present (aerobic): is present (aerobic):

Glycolysis is followed by three Glycolysis is followed by three pathways of cellular pathways of cellular respiration:respiration:• Pyruvate oxidationPyruvate oxidation• Citric acid cycleCitric acid cycle• Electron transport chainElectron transport chain

If OIf O22 is not present (anaerobic): is not present (anaerobic):

Pyruvate from glycolysis is Pyruvate from glycolysis is metabolized by fermentation.metabolized by fermentation.

Redox ReactionsRedox ReactionsRedox reactions: Redox reactions: One substance transfers One substance transfers

electrons to another substanceelectrons to another substanceReduction:Reduction: Gain of one or more electrons Gain of one or more electrons

by an atom, ion, or moleculeby an atom, ion, or moleculeOxidation:Oxidation: Loss of one or more electrons Loss of one or more electrons

Also occurs if Also occurs if hydrogenhydrogen atomsatoms are gained or are gained or lost (H = Hlost (H = H++ + e + e--))

Redox reactionsRedox reactionsOxidation and reduction Oxidation and reduction always occur togetheralways occur together..

The reactant that The reactant that becomes reduced is the becomes reduced is the oxidizing agentoxidizing agent..

The reactant that The reactant that becomes oxidized is the becomes oxidized is the reducing agentreducing agent..

Redox Reactions and GlucoseRedox Reactions and GlucoseIn glucose combustion, In glucose combustion, glucose is the reducing glucose is the reducing agent, Oagent, O22 is the oxidizing is the oxidizing agent.agent.

Energy is transferred in a Energy is transferred in a redox reaction.redox reaction.

Energy in the reducing agent Energy in the reducing agent (glucose) is transferred to (glucose) is transferred to the reduced product.the reduced product.

Electron CarriersElectron CarriersExamples: NAD, Examples: NAD,

FADH2, NADPHFADH2, NADPH

Coenzyme Coenzyme NADNAD++ is a is a key electron carrier key electron carrier in redox reactions.in redox reactions.

Two forms:Two forms:NADNAD++ (oxidized) (oxidized)NADH (reduced)NADH (reduced)

Complete vs. Incomplete Complete vs. Incomplete Oxidation of GlucoseOxidation of Glucose

Complete Oxidation

Incomplete Oxidation

The five metabolic pathways occur The five metabolic pathways occur in different parts of the cell.in different parts of the cell.

GlycolysisGlycolysis

Takes place in the cytosolTakes place in the cytosolConverts glucose into pyruvateConverts glucose into pyruvateProduces a small amount of energyProduces a small amount of energyGenerates no COGenerates no CO22

GlycolysisGlycolysisInvolves Involves tenten enzyme-catalyzed reactions. enzyme-catalyzed reactions.Energy-investing reactionsEnergy-investing reactions: Require ATP: Require ATPEnergy-harvesting reactionsEnergy-harvesting reactions: Yield NADH and ATP.: Yield NADH and ATP.

Results (net per glucose):Results (net per glucose):2 molecules of pyruvate2 molecules of pyruvate2 molecules of ATP2 molecules of ATP2 molecules of NADH2 molecules of NADH

““Energy Investing” Energy Investing” Steps of GlycolysisSteps of Glycolysis

So what is a kinase?So what is a kinase?

““Energy Harvesting” Energy Harvesting” Steps of GlycolysisSteps of Glycolysis

Substrate Level PhosphorylationSubstrate Level Phosphorylation

Enzyme-catalyzed transfer of a phosphate group Enzyme-catalyzed transfer of a phosphate group from a donor to ADP to form ATP is called from a donor to ADP to form ATP is called substrate-level phosphorylation.substrate-level phosphorylation.

Phosphorylation: addition of a phosphate group.Phosphorylation: addition of a phosphate group.

Anaerobic ConditionsAnaerobic ConditionsWithout OWithout O22, ATP can be produced by , ATP can be produced by

glycolysis and fermentation.glycolysis and fermentation.

Fermentation occurs in the cytosol, to Fermentation occurs in the cytosol, to regenerate NADregenerate NAD++. .

Pyruvate from glycolysis is reduced by Pyruvate from glycolysis is reduced by NADH + HNADH + H+.+.

Lactic Acid FermentationLactic Acid Fermentation• Occurs in Occurs in

microorganisms, microorganisms, some muscle cellssome muscle cells

• Pyruvate is the final Pyruvate is the final electron acceptorelectron acceptor

• Lactate is the Lactate is the product and can product and can build upbuild up

Alcohol FermentationAlcohol Fermentation• Requires two Requires two

enzymes to enzymes to metabolize pyruvate metabolize pyruvate to ethanolto ethanol

• Acetaldehyde is Acetaldehyde is reduced by NADH + reduced by NADH + HH++, producing NAD, producing NAD++ and glycolysis and glycolysis continuescontinues

Summary of Anaerobic Summary of Anaerobic Respiration Respiration

Cellular respiration yields more energy Cellular respiration yields more energy than fermentation per glucose than fermentation per glucose molecule.molecule.

• Glycolysis plus fermentation = 2 ATPGlycolysis plus fermentation = 2 ATP• Glycolysis plus cellular respiration = 32 Glycolysis plus cellular respiration = 32

ATPATP• So why do it?So why do it?

Aerobic Respiration: Pyruvate Oxidation Aerobic Respiration: Pyruvate Oxidation

Links Links glycolysisglycolysis and the and the citric acid cyclecitric acid cycle; ; occurs in the occurs in the mitochondrial matrixmitochondrial matrix

Pyruvate is oxidized to acetate and COPyruvate is oxidized to acetate and CO22 is is releasedreleased

NADNAD++ is reduced to NADH, capturing energy is reduced to NADH, capturing energySome energy is stored by combining acetate Some energy is stored by combining acetate

and Coenzyme A (CoA) to form acetyl CoA and Coenzyme A (CoA) to form acetyl CoA

Pyruvate OxidationPyruvate Oxidation

Per Glucose:2 NADH2CO2

2 Acetyl-CoA

Citric Acid CycleCitric Acid Cycle

InputsInputs:: acetyl CoA, water and electron acetyl CoA, water and electron carriers NADcarriers NAD++, FAD, and GDP, FAD, and GDP

Energy released is captured by Energy released is captured by ADPADP and and electron carriers electron carriers NADNAD++, FAD, and GDP, FAD, and GDP

OutputsOutputs:: CO CO22, reduced electron carriers, , reduced electron carriers, and ATP (really GTP)and ATP (really GTP)

Citric Acid CycleCitric Acid Cycle

The citric acid cycle is in steady state: The citric acid cycle is in steady state: The concentrations of the The concentrations of the intermediates don’t change.intermediates don’t change.

The cycle continues when starting The cycle continues when starting materials are available:materials are available:

• Acetyl CoAAcetyl CoA• Reoxidized electron carriersReoxidized electron carriers

Citric Acid CycleCitric Acid Cycle

Per Glucose:6 NADH2 FADH2

4 CO2

2 ATP

Complete Oxidation of Complete Oxidation of GlucoseGlucose

Recycle Electron CarriersRecycle Electron Carriers

The electron carriers that are reduced The electron carriers that are reduced during the citric acid cycle must be during the citric acid cycle must be reoxidizedreoxidized to take part in the cycle to take part in the cycle again.again.

Fermentation—if no OFermentation—if no O22 is present is present

Oxidative phosphorylationOxidative phosphorylation—O—O22 is present is present

Oxidative PhosphorylationOxidative Phosphorylation

Oxidative phosphorylation: ATP is Oxidative phosphorylation: ATP is synthesized by reoxidation of electron synthesized by reoxidation of electron carriers in the presence of Ocarriers in the presence of O22. .

Two stages:Two stages:• Electron transportElectron transport• Chemiosmosis Chemiosmosis

Electron Transport Chain Electron Transport Chain (ETC)(ETC)

Electrons from Electrons from NADH and FADHNADH and FADH22 pass pass through the respiratory chain of through the respiratory chain of membrane-associated carriers.membrane-associated carriers.Electron flow results in a Electron flow results in a proton proton concentrationconcentration (Membrane Potential) (Membrane Potential) gradient in mitochondria.gradient in mitochondria.

ETCETC

The respiratory chain is located in the The respiratory chain is located in the inner mitochondrial membrane inner mitochondrial membrane (cristae)(cristae)..

Energy is released as electrons are Energy is released as electrons are passed between carriers.passed between carriers.

Examples: protein complexes I, II, III, IV; Examples: protein complexes I, II, III, IV; Cytochrome Cytochrome c, c, ubiquinone (Q) ubiquinone (Q)

ETCETC

Proton Motive Force Proton Motive Force “Membrane Potential” “Membrane Potential”

During electron transportDuring electron transport protonsprotons are are also actively transported.also actively transported.

Protons accumulate in the Protons accumulate in the intermembrane space and create a intermembrane space and create a concentration gradient and charge concentration gradient and charge differencedifference——potential energy!potential energy!

This proton-motive force drives protons This proton-motive force drives protons back across the membrane.back across the membrane.

Proton Motive ForceProton Motive Force

I, III, and IV Pump Protons

ChemiosmosisChemiosmosis

Protons diffuse back into the Protons diffuse back into the mitochondria through ATP synthase, a mitochondria through ATP synthase, a channel protein.channel protein.

Diffusion is coupled to ATP synthesis.Diffusion is coupled to ATP synthesis.Oxidative PhosphorylationOxidative Phosphorylation is one is one

example of Chemiosmosisexample of Chemiosmosis

ATP SynthaseATP SynthaseFF00 subunit subunit: :

transmembranetransmembraneFF11 subunit subunit: projects : projects

into the into the mitochondrial mitochondrial matrix, rotates to matrix, rotates to expose active sites expose active sites for ATP synthesisfor ATP synthesis

Summary of Aerobic Summary of Aerobic RespirationRespiration

GlycolysisGlycolysis4 ATP – 2ATP used =2 ATP NET4 ATP – 2ATP used =2 ATP NET2 NADH2 NADH(IF FERMENTATION use 2 NADH to (IF FERMENTATION use 2 NADH to reduce pyruvate)reduce pyruvate)Pyruvate oxidationPyruvate oxidation2 NADH2 NADH2 CO2 2 CO2 TCA CycleTCA Cycle2 ATP2 ATP6 NADH6 NADH4 CO24 CO22 FADH22 FADH2

ETCETCNADH (TOTAL=10 x 2.5=25 ATP)NADH (TOTAL=10 x 2.5=25 ATP)FADH2 (2 x 1.5= 3 ATP)FADH2 (2 x 1.5= 3 ATP)Total ATP = 28Total ATP = 28

Total for Cell RespirationTotal for Cell Respiration4 ATP by substrate level 4 ATP by substrate level respirationrespiration28 ATP by oxidative 28 ATP by oxidative PhosphorylationPhosphorylation= 32 ATP= 32 ATP